1. Field of the Invention
The present invention relates to a ferrite sintered compact, and more particularly to a ferrite sintered compact capable of cutting unnecessary radiant noise in a high frequency (which may be referred to as xe2x80x9ca ferrite sintered compact for absorbing high frequency radiant noisexe2x80x9d in some cases).
2. Description of the Prior Art
Recently, with rapid progression of the information communication technology or pervasion of various kinds of electronic components, there has been a problem that unnecessary electromagnetic noise adversely affects the various electronic devices.
In general, ferrite is used for an EMI (electromagnetic interference) measure for cutting the unnecessary radiant noise of a target frequency by using an impedance characteristic. The impedance of the ferrite is constituted by a complex ratio permeability, and the impedance per one turn is represented by the following expression.
Z=xcexcxe2x80x3Axcfx89+ixcexcxe2x80x2Axcfx89
(Z: impedance, xcexcxe2x80x2: real part of the complex ratio permeability, xcexcxe2x80x3: imaginary part of the complex ratio permeability, A: geometric factor of a magnetic body, xcfx89: angular frequency).
In particular, since a frequency of a base clock is not less than 55 MHz and a clock frequency of a CPU is not less than 450 MHz, a frequency band to be emphasized when the ferrite is used for the EMI measure is 50 to 1000 MHz. Therefore, the high impedance is required from approximately 50 MHz in the ferrite.
As the prior art ferrite capable of cutting off the unnecessary radiant noise in such a high frequency, NiCuZn ferrite has been used.
The NiCuZn ferrite having the above-described characteristic, however, has such a problem as that an inexpensive ferrite sintered compact can not be provided since an NiO material is expensive. There is the move such that the MgCuZn ferrite or the MnMgCuZn ferrite using Mg(OH)2, MgO or MgCO3 which is cheaper than the NiO material is manufactured and the thus obtained product is utilized as the ferrite sintered compact capable of cutting off the unnecessary radiant noise in the high frequency. As a prior art technique concerning such a ferrite sintered compact, for example, Japanese patent gazette No. 2727403 and Japanese patent application laid-open No. 324564-1998 can be cited.
However, the capability for cutting off the radiant noise cannot be satisfactory even if the proposed technique according to the above gazette is used as it is, and a desired characteristic to reach a level that the present inventors intend is not obtained.
For example, in the technique disclosed in Japanese patent gazette No. 2747403 and Japanese patent application laid-open No. 324564-1998, an initial permeability is used as a reference for a method for controlling the impedance. However, when using the MgCuZn ferrite or the MnMgZn ferrite substituting Mn or Cu as described in an article, Journal of the Japan Society of Powder and Powder Metallurgy 42 (1995), 22 and another article, Journal of the Japan Society of Powder and Powder Metallurgy 43 (1996), 1399, since a relaxation phenomenon, i.e., reduction in the initial permeability involved by increase in the frequency occurs, the impedance of a high frequency can not necessarily be controlled with the initial permeability.
As described above, the impedance of the ferrite is constituted by a complex ratio permeability and the complex ratio permeability is substantially affected by the magnetization spin and the domain wall displacement. Therefore, it is considered that a parameter concerning the magnetization spin and the domain wall displacement must be found to be controlled in order to efficiently cut off the radiant noise.
Additionally, as described above, the impedance of the ferrite has not only the complex ratio permeability but also an advantage of a magnetic body shape. Thus, in order to value a ferrite sintered compact for efficiently cutting off the radiant noise, it can be considered that measuring the complex ratio permeability as a material constant which is not affected by the shape is preferable.
The present invention is made under such actuality and an object thereof is to provide a ferrite sintered compact which can have a high impedance with a frequency of not less than 50 MHz by selecting an inexpensive material and efficiently cut off radiant noise.
In order to solve such a problem, the present invention provides an MgCuZn-based ferrite sintered compact constituted by using 7.5 to 23.0 mole % magnesium oxide, 7.0 to 20.0 mole % of copper oxide, 19.0 to 24.2 mole % of zinc oxide, and 48.5 to 50.3 mole % of ferric oxide as substantially main component compositions, wherein an average particle diameter of the ferrite sintered compact is in a range of 1.10 to 7.30 xcexcm and a standard deviation "sgr" of a size distribution is in a range of 0.60 to 10.0.
As a preferable mode, the ferrite sintered compact according to the present invention is constituted such that a real part xcexcxe2x80x2 of a complex permeability is not less than 85 and an imaginary part xcexcxe2x80x3 of the complex permeability is not more than 170 with a frequency of 30 MHz and a temperature of 25xc2x0 C.; the real part xcexcxe2x80x2 of the complex permeability is not less than 45 and the imaginary part xcexcxe2x80x3 of the complex permeability is not less than 100 with a frequency of 50 MHz and a temperature of 25xc2x0 C.; and the real part xcexcxe2x80x2 of the complex permeability is not less than xe2x88x921.8 and the imaginary part xcexcxe2x80x3 of the complex permeability is not less than 16 with a frequency of 500 MHz and a temperature of 25xc2x0 C.
As another preferred mode, the ferrite sintered compact according to the present invention is constituted such that a real part xcexcxe2x80x2 of a complex permeability is not less than 15 and an imaginary part xcexcxe2x80x3 of the complex permeability is not less than 71 with a frequency of 100 MHz and a temperature of 25xc2x0 C.; and the real part xcexcxe2x80x2 of the complex permeability is not less than xe2x88x920.3 and the imaginary part xcexcxe2x80x3 of the complex permeability is not less than 27 with a frequency of 300 MHz and a temperature of 25xc2x0 C.
As still another preferred mode, the ferrite sintered compact according to the present invention is constituted in such a manner that it is fired at a firing temperature of less than 1200xc2x0 C.